Mills was born on 21 July 1944 in Tarkwa, in the Western Region of Ghana. He was from the town Ekumfi Otuam and was a member of the Fante people. He was educated at Achimota School, where he completed the Advanced-Level Certificate in 1963, and the University of Ghana, Legon, where he completed a law degree in 1967.
Mills studied at the London School of Economics and Political Science and earned a PhD in Law at the School of Oriental and African Studies at the University of London after completing his doctoral thesis in the field of taxation and economic development. Even during his career in politics he was sometimes called by the nickname "The Prof"
Mills' first formal teaching assignment was as a lecturer at the Faculty of Law at the University of Ghana. He spent close to twenty five years teaching at Legon and other institutions of higher learning. In 1971, he was selected for the Fulbright Scholar programme at Stanford Law School in the US.
He returned to Ghana after receiving his JD to work at his alma mater, the University of Ghana, for 25 years. He became a visiting professor of Temple Law School (Philadelphia, USA), with two stints from 1978 to 1979, and 1986 to 1987, and was a visiting professor at Leiden University (Holland) from 1985 to 1986. During this period, he authored several publications relating to taxation during the 1970s and 1980s.
Outside of his academic pursuits, Professor Mills was the Acting Commissioner of Ghana's Internal Revenue Service from 1986 to 1993 under President Jerry John Rawlings, and the substantive Commissioner from 1993 to 1996. By 1992, he had become an Associate Professor of Law at the University of Ghana.
Vice-President of Ghana
For the inaugural presidential election in 1992, the National Convention Party (NCP) had formed an alliance with the National Democratic Congress (NDC). Former Provisional National Defence Council (PNDC) Chairman, and leader of Ghana, Flight-Lieutenant Jerry John Rawlings chose the NCP leader, Kow Nkensen Arkaah, as his running-mate for vice-president. Having been elected in the 1992 election, Arkaah served between 1992–1996However, on 29 January 1996, the NCP broke with the NDC, merging with the People's Convention Party (PCP) to form a rebirth of the Convention People's Party. Arkaah stood as candidate for the reborn CPP in the 1996 presidential election against Rawlings. Rawlings selected Mills for the vacated Vice-Presidency in his bid for re-election to a second term in the election and was re-elected to his second term in office, serving from 1996 to 2000.
In 2000, Mills became the NDC's candidate for the 2000 presidential election after Rawlings had served his constitutionally mandated terms as president. The main rival for Mills' own bid for the presidency was John Agyekum Kufuor, who was running as the candidate for the opposition New Patriotic Party (NPP). In the first round, held on 7 December 2000, Mills gained 44.8% of the vote, Kufuor won the first round with 48.4%, thus forcing a second round. On 28 December 2000, Kufuor defeated Mills with 56.9% of the vote and was sworn in as president on 7 January 2001. It was during this time that the term Better Ghana Agenda was coined.
In December 2002, Mills was elected by his party to be its flag bearer and lead them into the 2004 election He was, however, defeated again by incumbent president John Agyekum Kufuor, who received 52.45% of the vote on the first ballot.
President of Ghana
On 21 December 2006, he became the NDC's candidate for the 2008 presidential election, winning his party's ticket by an 81.4% result (1,362 votes), far ahead of his opponents, Ekwow Spio-Garbrah, Alhaji Mahama Iddrisu, and Eddie Annan. In the 2008 election, John Agyekum Kufuor was no longer eligible to run as president, having served two terms. Mills' main opponent from the New Patriotic Party was now Nana Akufo-Addo. Mills ran under the campaign slogal of "A Better Man for a Better Ghana," on a platform of change. He said: "People are complaining. They're saying that their standard of living has deteriorated these past eight years. So if Ghana is a model of growth, it's not translating into something people can feel." The result of the first ballot had Akufo-Addo in front with 49.13% of the vote to Mills' 47.92%, however, a run-off second round of voting was needed. The second round of voting took place on 28 December 2008.The result was a slim lead held by Mills, but due to problems with the distribution of ballots, the Tain constituency, located in the Brong-Ahafo Region, was forced to vote again on 2 January 2009. The final result was a victory by Mills with 50.23% of the vote to Akufo-Addo's 49.77%. Mills became the third president of the 4th Republic Of Ghana.
He was married to Ernestina Naadu Mills, an educator, and had a son, Sam Kofi Atta Mills.
He was a good friend to T. B. Joshua, self-proclaimed "prophet" of The Synagogue, Church Of All Nations in Lagos, Nigeria and regularly visited his church. He said, following his inauguration, that Joshua had prophesied that it would take him three elections to win the presidency and that the result would be released in January.
He contributed to the Ghana Hockey Association, National Sports Council of Ghana and Accra Hearts of Oak Sporting Club. He enjoyed field hockey and swimming, and once played for the national hockey team (he remained a member of the Veterans Hockey Team until his death). He was also a board member of Hearts of Oak and a Manchester United fan.